Concrete classes can have abstract type members

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The following compiles:

class Baz { type T; }

I'm not 100% sure I'm not missing something in the SLS about default of type members, but I can't find anything.

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Imported From: https://issues.scala-lang.org/browse/SI-1753?orig=1
Reporter: @DRMacIver

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Geoffrey Alan Washburn (washburn) said:
This is an intentional "feature" Martin added before 2.7.2 or 2.7.3 (I can't remember which) but was not announced or as you note described in the SLS.

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@DRMacIver said:
Ok. Thanks.

I find it rather surprising (I found it during a presentation while explaining abstract type members, and both Miles and I were baffled), but will assume there's a good reason for it. :-)

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@milessabin said:
Good reason or not, there's some very strange behaviour here,

Welcome to Scala version 2.7.3.final (Java HotSpot(TM) Server VM, Java 1.6.0_10).
Type in expressions to have them evaluated.
Type :help for more information.

scala> class C { type T } 
defined class C

scala> object O extends C
defined module O

scala> var v1 : O.T = _
v1: O.T = null

scala> val v2 : Any = v1
v2: Any = null

scala> val v3 : AnyVal = v1
<console>:7: error: type mismatch;
 found   : O.T
 required: AnyVal
       val v3 : AnyVal = v
                         ^

scala> val v4 : AnyRef = v1
<console>:7: error: type mismatch;
 found   : O.T
 required: AnyRef
Note that implicit conversions are not applicable because they are ambiguous:
 both method any2stringadd in object Predef of type (Any)scala.runtime.StringAdd
 and method any2ArrowAssoc in object Predef of type [A](A)ArrowAssoc[A]
 are possible conversion functions from O.T to AnyRef
       val v4 : AnyRef = v
                         ^

So T is <: of Any, but not of AnyVal or AnyRef, even though null inhabits T?

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@odersky said:
So T is <: of Any, but not of AnyVal? or AnyRef?

yes.

even though null inhabits
T?

Null inhabits T only because you sneakily forgot to initialize the
value. You won't be able to assign null to a variable of type T (try
it!). It's the best one can do with Java `default' initializers,
except for an explicit initialization check. That would probably be
best, but it's not yet implemented.

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@ingoem said:
I leave this open until the spec is updated and reassign to Martin then.

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@odersky said:
I updated the spec.

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@Blaisorblade said:
While looking at #8217, I could actually find the rule in the spec in 5.1: "If the template forms part of a concrete class definition, stats may still contain declarations of abstract type members, but not of abstract term members."

However, this does not describe why it is forbidden to have:

object Baz { type T }

See #8217 for that.